Bolster Plate Assembly

ABSTRACT

A bolster plate is attached to a printed circuit board and acts as a stiffener that reduces bending in the overall assembly during the attachment of an integrated circuit chip to the printed circuit board under a heavy applied load. The bolster plate is provided with a shim that compensates for bending of the bolster plate under load, thereby preventing damage to the integrated circuit chip. The dimensions of the shim may be selected according to computer model results representing bow deformation in the bolster plate without the shim.

RELATED APPLICATIONS

This application is a divisional of U.S. patent Ser. No. 09/918,764,filed Jul. 30, 2001, now U.S. Pat. No. 6,789,312 entitled A METHOD OFATTACHING AN INTEGRATED CIRCUIT TO A CHIP MOUNTING RECEPTACLE IN A PCBWITH A BOLSTER PLATE, the aforementioned application is incorporatedherein by reference thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to the field of electrical equipment manufactureand, particularly, the electrical attachment of integrated circuitchips, such as microprocessors and application-specific integratedcircuits (ASICS), to printed circuit boards. More specifically, themethod of attachment uses a bolster plate to reinforce the printedcircuit board during the attachment process.

2. Discussion of the Related Art

A variety of methods have been devised for the attachment of integratedcircuit chips, such as microprocessors and ASCIS, to printed circuitboards in a manner that assures consistency in establishing goodelectrical contact. Many of these methods involve the use of highcompressive loads, which are applied to the processors. The integratedcircuit chips typically have a plurality of pins that mate with acorresponding female conductive receptacle in the printed circuit board.The printed circuit boards, alone, lack sufficient rigidity to supportthe compressive loads during attachment. For example, these loads mayrange from two hundred to three hundred pounds force (890 to 1300Newtons). Resultant bending of the printed circuit board is capable ofdamaging wiring or other materials within the integrated circuit chip.Additionally, the bending moment is capable of disrupting the desiredelectrical contact.

The problem of printed circuit board bending under these heavy loads istypically resolved by using a bolster plate, which is a piece of metalthat attaches to the printed circuit board, e.g., by bolting, riveting,or adhesion. The bolster plate may be constructed in any geometricalshape that provides the requisite support. The bolster plate is usuallylocated on the reverse side of the printed circuit board opposite thatside on which the integrated circuit chip resides.

Newer microprocessors and ASICS devices have increased numbers of pinsin comparison to older devices. Furthermore, the newer devices operateat much higher speeds than did older devices. The increasing number ofpins and higher levels of performance demand closer mechanicaltolerances for manufacturing purposes. It has been discovered that theuse of a bolster plate according to traditional practices does notsufficiently eliminate the bending moment in the printed circuit boardsin light of these new demands. In applications where, for example, aforce of 270 pounds (1200 Newtons) is applied to seat a microprocessor,a conventional bolster plate may bow a distance of 0.001 inch (0.0025cm). Even this small amount of bending is sufficient damage the assemblyor to cause failure in the electrical contact.

The bolster bow or bend is at maximum in the center of the bolsterplate. Additional rigidity could be imparted by increasing the thicknessof the bolster plate, but this requires additional room for the bolsterplate. The increased thickness creates other difficulties in the contextof fitting additional components on the printed circuit board and inassembling adjacent components in the intended use environment. Attemptshave been made to pre-bow or pre-stress the bolster plate to accommodatethe stress during the attachment of microprocessors, but the resultingbending moment from pre-stressing the bolster plate was not repeatable.

There remains a problem in preventing bolster plate bending due to theinsertion of newer microprocessors and ASICS devices.

SUMMARY OF THE INVENTION

A bolster plate according to the principles described herein overcomesthe problems described above and advances the art by providing a method,apparatus and software pertaining to a shim or shim assembly thatcompensates the bolster plate for bending deformations during theattachment of integrated circuit chips. The shim may be located, forexample, at a position where the maximum amount of deformation occursunder load from the attachment process. Thus, the shim substantiallyfills the deformation under load and prevents damage to themicroprocessor by providing support to the assembly preventingcorresponding deformation in the integrated circuit chip, notably, inthe pins, wiring and silicon, which are subject to breakage under smallamounts of deformation.

A bolster plate according to these principles is used for supporting aprinted circuit board during attachment of an integrated circuit chip tothe printed circuit board. The bolster plate comprises a support railpresenting a contact face for use in contacting the printed circuitboard. The rail demarcates a central well that contains a platformpresenting a support surface configured to support a selected portion ofthe printed circuit board underneath the integrated circuit chip duringattachment of the integrated circuit chip to the printed circuit board.Where the bolster plate is made out of a metal, an insulator preferablycovers the support surface. A shim is interposed between the insulatorand the support surface where the insulator is required to prevent shortcircuiting of the integrated circuit chip. With or without theinsulator, the shim is positioned at a point or points of maximumdeformation in the bolster plate. The dimensions of the shim arepreselected to compensate for deformation of the bolster plate under thedesign load by filling the point or points of maximum deformation.

While the dimensions of the shim may be determined by trial and error, amuch preferred manner of determining the shim dimensions is tocalculate, e.g., through finite element mathematical modeling, thepredetermined dimensions that are operable to compensate for bending ofthe bolster plate under a maximum applied load during attachment of theintegrated circuit chip. This modeling assures that the deformed supportsurface under load is shim-compensated to present a total deformation ofless than, for example, a 0.001 inch or 0.0005 inch (0.0025 or 0.0038cm) bow at a center of the bolster plate under the maximum applied load.The term “finite element modeling” is hereby defined to include bothfinite analysis and finite difference modeling techniques.

The shim may have any geometrical shape, such as a square or rectangularshape, but a disk or ovaloid shape is preferred for correspondence withthe shape of bow deformation in the bolster plate. Particularlypreferred shims comprise a plurality of pieces, such as two disks, wherethe pieces have different dimensions and are concentrically stacked topresent a stair-stepped edge providing a transition to the supportsurface that is less abrupt than a non-tapered shim. Alternatively, asingle shim may be tapered or machined to have a stair-step, in order toease the transition.

The bolster plate that is described above may be used in a method ofattaching an integrated circuit chip to a chip-mounting receptacle in aprinted circuit board. The method comprises the steps of assembling abolster plate including the shim, attaching the bolster plate to theprinted circuit board; and pressing the integrated circuit chip into thechip mounting receptacle. Further method steps preferably but optionallycomprises modeling a bending moment in the bolster plate under a maximumapplied load for use in the step of pressing the integrated circuit chipto provide model results for shim-based compensation of the bendingmoment, and selecting dimensions of the shim based upon the modelresults.

The principles described herein also pertain to a computer readable formcomprising machine instructions that are operable for determining a bowdeformation in the bolster plate when the bolster plate is placed undera maximum load during attachment of an integrated circuit chip, andidentifying dimensions for the shim that may be used to compensate forthe bow deformation. In a manufacturing environment, permits theselective adjustment of shim dimensions to compensate for bowdeformation on the basis of different bolster plate designs andmaterials, as well as different applied loads. If an increase in devicefailure rate is traceable to the chip attachment process, the shimdimensions can be selectively adjusted to overcome the observed failureincreases.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an assembly view depicting attachment of a microprocessor to aprinted circuit board with use of a bolster plate;

FIG. 2 is a top side view of a printed circuit board having achip-mounting receptacle;

FIG. 3 is a bottom side view of the printed circuit board shown in FIG.2, and shows a bolster plate installed underneath the chip-mountingreceptacle;

FIG. 4 is a top perspective view of a bolster plate providing additionaldetail with respect to the bolster plate shown in FIG. 3;

FIG. 5 is a top perspective view of the bolster plate from FIG. 4,further including a shim placed in the center of a support surface;

FIG. 6 is a top perspective view of the bolster plate shown in FIG. 5after installation of an insulating layer to cover the shim and thesupport surface;

FIG. 7 is a midsectional view of the bolster plate in unstressedcondition taken along line 7–7′ of FIG. 6;

FIG. 8 depicts the bolster plate midsectional view of FIG. 7 under aheavy applied load; and

FIG. 9 depicts a process diagram for use in attaching an integratedcircuit chip to a printed circuit board with use of the bolster plate.

DETAILED DESCRIPTION OF THE EMBODIMENTS

There will now be shown by way of example and not by limitation, abolster plate in use for supporting a printed circuit board duringattachment of an integrated circuit chip to the printed circuit board.The bolster plate comprises a support rail presenting a contact face foruse in contacting the printed circuit board. The rail demarcates acentral well that contains a support surface configured to support aselected portion of the printed circuit board underneath the integratedcircuit chip during attachment of the integrated circuit chip to theprinted circuit board.

FIG. 1 is an assembly view showing an integrated circuit chip 100, e.g.,a microprocessor, that is being attached to a printed circuit board 102by insertion into a chip socket 200. A heat sink assembly 106 comprisesa central electric fan 108 with a corresponding power coupling 110, andwell as heat-conductive fins 112. A square base plate 114 at each cornercontains a shoulder screw, such as screw 116, that is circumscribed by acompression spring, such as compression spring 118.

The shoulder screws, e.g., shoulder screw 116, are each inserted intocorresponding holes 120, as well as threaded apertures, such as aperture420, in a bolster plate 300 that underlies the printed circuit board102. The heat sink assembly is lowered until the base plate 114 contactsintegrated circuit chip 100. Pins 130 are aligned with correspondingreceptacles 132. Gradual balanced tightening of the shoulder screws intothe bolster plate 300, e.g., as shoulder screw 116 is extended throughhole 120 and threaded into threaded aperture 420, then forces the pins130 fully into receptacles 132. The compression springs, e.g.,compression spring 118, are calibrated to place a uniform, permanentpredetermined compressive load 134 on base plate 114 and integratedcircuit chip 100 once the shoulder screws are fully and equallytightened. This load 134 may, for example, in combination from all ofthe compression springs range from 200 to 500 pounds force, and a loadof 270 pounds is preferred for the attachment of microprocessors.

FIG. 2 provides additional detail with respect to the printed circuitboard 102 shown in FIG. 1. FIG. 2 is a top side view of printed circuitboard 102 having the chip-mounting socket 200 comprising a plurality offemale pin receptacles 202, such as receptacle 132. A top face 206 maycontain any feature of printed circuit boards that facilitate designoperations of the printed circuit board 102. FIG. 2 does not show thesefeatures in detail, but they may include, for example, resistors,capacitors, inductors, additional integrated circuit devices, buses, andmetalized pathways that establish communication between thesecomponents.

FIG. 3 provides additional detail with respect to the printed circuitboard 102 shown in FIG. 1. FIG. 3 is a bottom side view of the printedcircuit board 102, and shows a bolster plate 300 installed underneaththe chip-mounting socket 200 (shown in phantom). A bottom face 302 mayoptionally contain any feature of printed circuit boards that facilitatedesign operations of the printed circuit board 102. FIG. 3 does not showthese features in detail, but they may include, for example, resistors,capacitors, inductors, additional integrated circuit devices, buses, andmetalized pathways that establish communication between thesecomponents. The use of a dual-sided board including a bottom face 302with these features installed increases the density of printed circuitboard 102.

FIG. 4 provides additional detail with respect to the bolster plate 300shown in FIG. 3. The bolster plate 300 is preferably stamped from asingle piece of metal to form rail 400 presenting flat contact surfaces402 and 404 that is adapted to fit flush against the bottom face 302(see FIG. 3) of printed circuit board 102. The rail 400 may bediscontinuous to present a plurality of flat contact surfaces, such assurfaces 402 and 404. Discontinuities, such as discontinuities 406 and408 in the rail 400 may be provided as a mater of design choice topermit the passage of metalization layers, electrically conductiveleads, or other components on the printed circuit board (not shown)without interference from the bolster plate 300. The bolster plate 300may also contain cavities, such as cavities 410, 412, 414, and 416, asneeded to permit the passage of components mounted on the printedcircuit boards, such as selected bottom side portions of the pinmounting socket 200 (see also FIGS. 2 and 3). The rail 400 comprises aplurality of apertures, such as apertures 418, 420, and 422, which maybe used to bolt or rivet the bolster plate 300 to the printed circuitboard 102, as shown in FIGS. 1.

The rail 400 substantially circumscribes a central square well 424,which contains a microprocessor support platform 426 presenting asupport surface 428. The support surface 428 is typically lower than thecontact surfaces 402 and 404 (as shown in FIG. 4), but may occupy thesame elevation as or be higher than the contact surfaces 402 and 404.

The bolster plate 300, as shown and described to this point, may be anytype of bolster plate for use on printed circuit boards. The dimensionsand structure of the bolster plate 300 may be any dimensions andstructure, as may be desired according to design choice. The specificgeometry of the bolster plate 300 is not necessarily critical, exceptthat the bolster plate must compliment the printed circuit board 102 formounting purposes and should have sufficient strength to fulfill itspurposes.

There will now be shown a modification to bolster plate 300, accordingto the preferred instrumentalities described herein, to enhance theutility of bolster plate 300 by using a shim to facilitate improvedsupport to a printed circuit board during attachment of an integratedcircuit chip to the printed circuit board. The bolster plate comprises asupport rail presenting a contact face for use in contacting the printedcircuit board. The rail demarcates a central well that contains asupport surface configured to support a selected portion of the printedcircuit board underneath the integrated circuit chip during attachmentof the integrated circuit chip to the printed circuit board. Where thebolster plate is made out of a metal, an insulator covers the supportsurface. The shim is interposed between the insulator and the supportsurface.

FIG. 5 depicts the bolster plate 300, exactly as shown and described inFIG. 4, with the addition of a shim 500 that is centrally located withrespect to support surface 428. The shim 500 may have any geometricalshape, such as a square, rectangle, triangle, or combination of shapes,such as a square or triangle with rounded corners. The optimumdimensions of the shim 500, such as width, length, and thickness, arepreferably determined by finite element modeling of the bending momentin bolster plate 300, as described below in additional detail.

As shown in FIG. 5, shim 500 comprises two separate disks 502 and 504.Disk 502 is in direct contact with support surface 428, and disk 504resides concentrically above disk 502. Disk 504 has a smaller diameterthan does disk 502, which presents a stair-step 506 or taper inprogression downward from disk 504 to disk 502 and support surface 428.Alternatively, a single-piece shim 500 may be used, additional disks ofincreasingly smaller diameter may be stacked atop disk 504, or a singletapered disk may be used. The disks 502 and 504 may be coated withadhesive on one or both sides to enhance their positional stabilityduring the assembly process.

FIG. 6 depicts the bolster plate 300 in final assembly as it ismade-ready for attachment to a printed circuit board (not shown). Thebolster plate 300, as shown in FIG. 6, is identical to the bolster plate300 as shown in FIG. 5, except a square insulator 600, preferably butoptionally with adhesive backing, has been placed in well 424 to coverthe shim 500 and support surface 428.

FIG. 7 is a midsectional view taken along line 7–7′ of FIG. 6. The scaleof FIG. 7 is exaggerated to show relatively increased thicknesses of theinsulator 600 and the shim 500 relative to other components. As shown inFIG. 7, the bolster plate 300 is in an unstressed state where the shim500 causes a bulge 700 to form in the middle of well 424.

FIG. 8 depicts the bolster plate 300 along the same midsectional viewshown in FIG. 7, however, the bolster plate 300 as shown in FIG. 8 isstressed by loading conditions, such as may be imposed by a maximumapplied load during use of the assembly shown in FIG. 1. A maximumapplied load 800 has induced a bending moment in the support platform426, such that the bulge 700 of FIG. 7 has been substantially eliminatedto present a flat surface. The bending moment causes buckling ordeformation of vertical magnitude D, which is the approximate thicknessof the shim 500. Where the magnitude of D is, for example, 0.001 inches,the availability of shim 500 reduces the magnitude of such bucklingpresented at surface 428 to a value less than 0.001 inches under thestatic or dynamic loading conditions that are imposed by the maximumapplied load 800. Due to the relatively small thickness of shim 500, itdoes not matter whether support platform is transiently or permanentlydeformed by the maximum applied load 800.

The dimensions of shim 500 vary depending upon the dimensions of bolsterplate 300 and the magnitude of applied load 800. The bolster plate 300may be any bolster plate that is designed for the support of anyintegrated circuit chip. Accordingly, no one set of dimensions in shim500 can be used to accommodate all applications. The deformation inbolster plate 300 may be observed by physical measurements without theshim 500 attached. The deformation may also be modeled by finite elementor finite difference techniques based upon the actual dimensions andmaterials that are used in bolster plate 300. The dimensions of shim 500may also be adjusted based upon experience-in-use factors. If, forexample, manufacturing processes result in failures of integratedcircuit chips due to breakage that is induced by the installationprocess, the dimensions of the shim 500 may be adjusted to provide moreor less support in the area of breakage depending upon the nature of thebreakage.

Along these lines, it should be noted that the dimensions of supporttable 426 are preselected as a matter of design choice in designing aconventional bolster plate. Designers, in choosing the dimensionsstructures like platform 426 and rail 400, normally intend to support acorresponding area underlying a selected portion of the overlying pinmounting receptacle 200, printed circuit board 102 and base plate 114,that is selected in the judgment of such designers as being needful ofsupport. Some circumstances may arise where the observed or modeleddeformation in bolster plate 300 is difficult to compensate with a shim500 due to complex geometrical constructions and alignment of parts. Inthese circumstances, a bolster plates may be designed to provide a lesscomplex deformation that can be easily compensated through use for ashim 500.

FIG. 9 is a process schematic diagram illustrating a preferred method900 for attaching an integrated circuit chip to a chip-mountingreceptacle in a printed circuit board with use of a bolster plate tosupport the printed circuit board. The method begins in step 902 withthe modeling of bolster plate deformation under an applied load. Avariety of commercially available finite element modeling packages maybe used for this purpose. Two such commercially available finite elementmodeling programs that are particularly preferred for use in modelingthe deformation of bolster plates, such as bolster plate 300, includeMECHANICA® and Pro/MECHANICA®, both of which are produced by ParametricTechnology Corporation of Waltham, Mass. Other packages, such as RASNA®,formerly produced by Rasna Corporation of San Jose, Calif., and avariety of other packages may also be programmed with data to model suchdeformations. The dimensions of the shim, such as shim 500, are intendedto compensate for the modeled deformations by filling the point orpoints of maximum deformation under the maximum applied load, as shownin FIG. 8.

Step 904 entails assembling the bolster plate, as shown for bolsterplate 300 in the context of FIGS. 3 through 6. The assembly, forexample, as described above, preferably includes a rail 400 thatprovides a face 402, 404 for use in contacting the printed circuit board102. The rail also demarcates a central well 424. The central wellcontains a support surface 428 configured to support a selected portionof the printed circuit board underneath the integrated circuit chipduring attachment of the integrated circuit chip to the printed circuitboard. An insulator 600 covers the support surface 428. A shim 500embodies dimensions corresponding to the model results of step 902 andis interposed between the insulator and the support surface.

Step 906 includes attaching the bolster plate to the printed circuitboard by any conventional means, such as bolting, riveting or adhesion.Step 908 includes pressing the integrated circuit chip into the chipmounting receptacle, e.g., as shown in FIG. 1.

Another aspect of the preferred instrumentalities described hereinpertains to a computer readable form comprising machine instructions.The instructions are operable for determining a bow deformation in abolster plate when the bolster plate is placed under a maximum loadduring attachment of an integrated circuit chip, and identifyingdimensions for a shim that may be used to compensate for the bowdeformation. This type of computer readable form may comprise a datafile or object containing data and program instructions in combinationwith one of the commercially available finite element modeling packagesdescribed above in the context of step 902, as shown in FIG. 9. Theprogram instructions may, for example, include instructions for theformation of a grid that is useful for finite element modeling,materials information, dimensions of the bolster plate, anditeration/convergence criteria.

The foregoing discussion is intended to illustrate the concepts of theinvention by way of example with emphasis upon the preferred embodimentsand instrumentalities. Accordingly, the disclosed embodiments andinstrumentalities are not exhaustive of all options or mannerisms forpracticing the disclosed principles of the invention. The inventorhereby states his intention to rely upon the Doctrine of Equivalents inprotecting the full scope and spirit of the invention.

1. A bolster plate assembly for use in supporting a printed circuitboard during attachment of an integrated circuit chip to the printedcircuit board, comprising: a support rail presenting a contact face foruse in contacting the printed circuit board; the rail demarcating acentral well; the central well containing a support surface configuredto support a selected area underneath the integrated circuit chip duringattachment of the integrated circuit chip to the printed circuit board;an insulator covering the support surface; and a shim interposed betweenthe insulator and the support surface, the shim contacting the supportsurface to receive support therefrom against compressive loading on theinsulator.
 2. The bolster plate assembly of claim 1, wherein the shimhas predetermined dimensions providing means for compensating forbending of the bolster plate under a maximum applied load duringattachment of the integrated circuit chip to the printed circuit board.3. The bolster plate assembly of claim 2, wherein the predetermineddimensions are provided according to a process of finite elementmodeling of bending characteristics of the bolster plate under themaximum applied load.
 4. The bolster plate assembly of claim 3, whereinthe predetermined dimensions are sufficient to impart less that 0.001inch bow at a center of the bolster plate under the maximum appliedload.
 5. The bolster plate assembly of claim 3, wherein thepredetermined dimensions are sufficient to impart less that 0.0005 inchbow at a center of the bolster plate under the maximum applied load. 6.The bolster plate assembly of claim 1, wherein the shim comprises adisk.
 7. The bolster plate assembly of claim 1, wherein the shimcomprises at least two different pieces of different dimensions.
 8. Thebolster plate assembly of claim 1, further comprising the integratedcircuit chip and the printed circuit board assembled to the bolsterplate.